North Atlantic (NA) deep-water formation and the resulting Atlantic meridional overturning cell is
generally regarded as the primary feature of the global overturning circulation and is believed to be a result
of the geometry of the continents. Here, instead, the overturning is viewed as a global energy–driven system
and the robustness of NA dominance is investigated within this framework. Using an idealized geometry
ocean general circulation model coupled to an energy moisture balance model, various climatic forcings are
tested for their effect on the strength and structure of the overturning circulation. Without winds or a high
vertical diffusivity, the ocean does not support deep convection. A supply of mechanical energy through
winds or mixing (purposefully included or due to numerical diffusion) starts the deep-water formation.
Once deep convection and overturning set in, the distribution of convection centers is determined by the
relative strength of the thermal and haline buoyancy forcing. In the most thermally dominant state (i.e.,
negligible salinity gradients), strong convection is shared among the NA, North Pacific (NP), and Southern
Ocean (SO), while near the haline limit, convection is restricted to the NA. The effect of a more vigorous
hydrological cycle is to produce stronger salinity gradients, favoring the haline state of NA dominance. In
contrast, a higher mean ocean temperature will increase the importance of temperature gradients because
the thermal expansion coefficient is higher in a warm ocean, leading to the thermally dominated state. An
increase in SO winds or global winds tends to weaken the salinity gradients, also pushing the ocean to the
thermal state. Paleoobservations of more distributed sinking in warmer climates in the past suggest that
mean ocean temperature and winds play a more important role than the hydrological cycle in the overturning
circulation over long time scales.

A growing number of paleoceanographic observations suggest that the ocean’s deep ventilation is stronger
in warm climates than in cold climates. Here we use a general ocean circulation model to test the hypothesis that
this relation is due to the reduced sensitivity of seawater density to temperature at low mean temperature; that is,
at lower temperatures the surface cooling is not as effective at densifying fresh polar waters and initiating
convection. In order to isolate this factor from other climate-related feedbacks we change the model ocean
temperature only where it is used to calculate the density (to which we refer below as ‘‘dynamic’’ temperature
change). We find that a dynamically cold ocean is globally less ventilated than a dynamically warm ocean. With
dynamic cooling, convection decreases markedly in regions that have strong haloclines (i.e., the Southern Ocean
and the North Pacific), while overturning increases in the North Atlantic, where the positive salinity buoyancy is
smallest among the polar regions. We propose that this opposite behavior of the North Atlantic to the Southern
Ocean and North Pacific is the result of an energy-constrained overturning.

We have previously argued that the Antarctic and subarctic North Pacific are
stratified during ice ages, causing to a large degree the observed low CO2 levels of
ice age atmospheres by sequestering respired CO2 in the ocean abyss. Here, we suggest
a mechanism for the major deglaciations of the late Pleistocene. The mechanism
begins with freshwater discharge to the North Atlantic, as evidenced by a
Heinrich event, that shuts down North Atlantic overturning. Because of a global
requirement for deep ocean ventilation, the North Atlantic shutdown drives overturning
in the Antarctic, which, in turn, releases CO2 to the atmosphere and reduces
Antarctic sea ice extent. The resulting increase in atmospheric CO2 and decrease in
albedo then drive global warming and deglaciation. As a control on the timing of
deglaciations, we look to the sensitivity of atmospheric freshwater transport to low
latitude temperature, which is a natural antagonist to Antarctic stratification under
cold climates. While Antarctic stratification is proposed to develop early in a glacial
period, continued cooling through the glacial period may reduce the poleward
atmospheric freshwater transport and thus may prepare the Antarctic halocline for
collapse. Deglaciations may coincide with obliquity maxima because a reduced
low-to-high latitude insolation gradient decreases the net poleward freshwater
transport and perhaps also because increased polar insolation can warm the deep
ocean and shift the westerly winds poleward, all of which should work to weaken
Antarctic stratification. Precession minima may encourage Antarctic destratification
by biasing tropical water vapor transport toward the northern hemisphere.
Finally, obliquity and precession may work together to encourage the circum-North
Atlantic freshwater discharge event that initiates the deglacial sequence.